Docker provider metrics cleanups and renaming
Instead of tracking action metrics separately, use direct Prometheus instrumentation for counting actions. Clean up metric names.
I removed the test that tests the specific number of time series, because it would have required pre-initializing all state label values on the actions counter to be meaningful. It's also not that useful to test the exact number of series returned by a type.
The machineProvider has to be constructed via newMachineProvider() now though (wasn't always happening in tests), as otherwise its metric metric members will not be initialized when using it.